Expected result:
Unity should not crash. Deleted prefab instance should be restored in Hierarchy panel.

Notes:
- Reproducible in 5.6.0b6;
- Was unable to reproduce in previous versions, since Editor crashes when opening project from previous versions;
- Was unable to repro from scratch; seems to be related to the current project;
- Reproducible on Windows 10 and OSX 10.12.2.